What do they say here? Are they implying that humic substances inhibit calcium carbonate?
Chemical Geology
Volume 201, Issues 1–2, 14 November 2003, Pages 91-101
The influence of dissolved humic acids on the kinetics of calcite precipitation from seawater solutions
Author links open overlay panelPierpaoloZuddasDamienFaivre
https://doi.org/10.1016/S0009-2541(03)00230-4Get rights and content
Abstract
The influence of dissolved organic matter on the complex mechanism of calcite crystal growth from seawater was evaluated by a set of experiments at different humic acid concentrations (i.e. [HA]=50, 500, 1000 μg/kg) in NaCl–CaCl2 solutions at a total ionic strength of 0.7 mol/kg. The temperature and PCO2 of the experiments were maintained at 298 K and 40 Pa, respectively. The constant addition technique was used in order to maintain [Ca2+] at ≅10.5 mmol/kg, while the [CO32−] was varied to isolate its role on the precipitation rate. Assuming that the calcite precipitation in this solution is dominated by the reaction:
(A1)Ca2++CO32−↔kbkfCaCO3" role="presentation" style="box-sizing: border-box; display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">Ca2++CO32−↔kbkfCaCO3
where kf and kb are the forward and reverse reaction rate constants, respectively, the net precipitation rate, R, can be described at any dissolved organic matter content by the difference between the forward and reverse rates:
(A2)R=kf(aCa2+)n1(aCO32−
n2−kb" role="presentation" style="box-sizing: border-box; display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">R=kf(aCa2+)n1(aCO32−)n2−kb
or, in its logarithmic form:
(A3)log(R+kb)−logKf+n2log[CO32−]" role="presentation" style="box-sizing: border-box; display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">log(R+kb)−logKf+n2log[CO32−]
where ni are the partial reaction orders with respect to the participating ions, a and γ are the ion activities and activity coefficients, respectively, and, Kf=kf(aCa2+)n1 (γCO32−)n2, a constant.
Results of this study indicate that, similarly to seawater and NaCl–CaCl2 solutions at the same ionic strength, the partial reaction order with respect to the carbonate ion concentration is 3, while the forward reaction rate constant, Kf, decreases by one order of magnitude when the dissolved organic matter concentration increased from 0 to 1000 μg/kg. This suggests that the mechanism of calcite precipitation is independent of the dissolved organic matter concentration even if such a component inhibits the calcite precipitation rate. Applying our model to previous rate measurements carried out in seawater solution under the compositional condition [Ca2+]≫[CO32−], we found that the rate of calcite precipitation from seawater solutions, a complex function of PCO2 and seawater inorganic inhibitors, still decreases as a function of the [HA] by at least one order of magnitude. Finally, we propose that the dissolved organic matter under the form of HA inhibits the calcite precipitation rate from seawater by covering the active growth sites rather than by complexation of calcium in solution.
Keywords
Kinetics
Calcite crystal growth
Organic matter
Seawater
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Copyright © 2003 Published by Elsevier B.V.
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Copyright © 2017 Elsevier B.V. or its licensors or contributors. ScienceDirect ® is a registered trademark of Elsevier B.V.
Chemical Geology
Volume 201, Issues 1–2, 14 November 2003, Pages 91-101
The influence of dissolved humic acids on the kinetics of calcite precipitation from seawater solutions
Author links open overlay panelPierpaoloZuddasDamienFaivre
https://doi.org/10.1016/S0009-2541(03)00230-4Get rights and content
Abstract
The influence of dissolved organic matter on the complex mechanism of calcite crystal growth from seawater was evaluated by a set of experiments at different humic acid concentrations (i.e. [HA]=50, 500, 1000 μg/kg) in NaCl–CaCl2 solutions at a total ionic strength of 0.7 mol/kg. The temperature and PCO2 of the experiments were maintained at 298 K and 40 Pa, respectively. The constant addition technique was used in order to maintain [Ca2+] at ≅10.5 mmol/kg, while the [CO32−] was varied to isolate its role on the precipitation rate. Assuming that the calcite precipitation in this solution is dominated by the reaction:
(A1)Ca2++CO32−↔kbkfCaCO3" role="presentation" style="box-sizing: border-box; display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">Ca2++CO32−↔kbkfCaCO3
where kf and kb are the forward and reverse reaction rate constants, respectively, the net precipitation rate, R, can be described at any dissolved organic matter content by the difference between the forward and reverse rates:
(A2)R=kf(aCa2+)n1(aCO32−
n2−kb" role="presentation" style="box-sizing: border-box; display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">R=kf(aCa2+)n1(aCO32−)n2−kbor, in its logarithmic form:
(A3)log(R+kb)−logKf+n2log[CO32−]" role="presentation" style="box-sizing: border-box; display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">log(R+kb)−logKf+n2log[CO32−]
where ni are the partial reaction orders with respect to the participating ions, a and γ are the ion activities and activity coefficients, respectively, and, Kf=kf(aCa2+)n1 (γCO32−)n2, a constant.
Results of this study indicate that, similarly to seawater and NaCl–CaCl2 solutions at the same ionic strength, the partial reaction order with respect to the carbonate ion concentration is 3, while the forward reaction rate constant, Kf, decreases by one order of magnitude when the dissolved organic matter concentration increased from 0 to 1000 μg/kg. This suggests that the mechanism of calcite precipitation is independent of the dissolved organic matter concentration even if such a component inhibits the calcite precipitation rate. Applying our model to previous rate measurements carried out in seawater solution under the compositional condition [Ca2+]≫[CO32−], we found that the rate of calcite precipitation from seawater solutions, a complex function of PCO2 and seawater inorganic inhibitors, still decreases as a function of the [HA] by at least one order of magnitude. Finally, we propose that the dissolved organic matter under the form of HA inhibits the calcite precipitation rate from seawater by covering the active growth sites rather than by complexation of calcium in solution.
Keywords
Kinetics
Calcite crystal growth
Organic matter
Seawater
Choose an option to locate/access this article:
Check if you have access through your login credentials or your institution.
Check Access
or
PurchaseRent at DeepDyve
or
Check for this article elsewhere
Copyright © 2003 Published by Elsevier B.V.
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Cookies are used by this site. For more information, visit the cookies page.
Copyright © 2017 Elsevier B.V. or its licensors or contributors. ScienceDirect ® is a registered trademark of Elsevier B.V.
